Quote:
Originally Posted by ike84
So I need to go and watch scanner danners videos about cats because my knowledge of them is still very much comic book version. When they fail what is the main symptom? Do they choke down the exhaust and kill power through reduction of flow? Or do they just stop cleaning up the exhaust and result in no other difference? I remember newarts video of how he toasted his cats when he blew his engine but that's the only other time I've ever heard of them going bad.
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Yes, ScannerDanner and also Schrodingers Box on youtube have great videos on fuel trims, O2 sensors, failing cats etc. ScannerDanner is a professional and Schrodingers Box is a DIY guy, so the videos are different, but both focus on the mathematics and science of engine controls.
From what I've learned watching their videos it appears that your former statement is what occurs. The failed cats typically impede the flow and result in very poor engine response and a lot of hot start issues. That said, I see exactly what blue62 was referring to with respect to my downstream O2 sensors. With the car at a steady rpm, a properly functioning cat should result in a downstream O2 sensor with a very flat output. My downstream O2 sensors do not look like they are doing that. Hence, a back-pressure test, to see if the cats are clogged or failing ... or at least that's my understanding from watching ScannerDanner and asking questions of my mechanic buddy.